1. Field of the Invention
The present invention relates to a metering device and more particularly to a device for providing fine control of the movement of a capillary tube. The invention provides very fine control of the rate of aspiration of a fluid flowing through the capillary tip.
2. Background of the Invention
In the field of devices for controlling the aspiration rate of a fluid, it is common to use a capillary tube through which fluid is drawn by an air stream under pressure which creates a vacuum. Such devices operate on the Venturi Principle wherein a stream of air is vented at a predetermined pressure level at the capillary tip in a direction perpendicular to the axis of the capillary tube. The opposite end of the capillary tube is injected into a solution. When the air stream passes the capillary tip, it creates a vacuum which draws the fluid through the capillary tube and aspirates it.
Such devices are typically used in flame photometers such as the kind manufactured by Instrumentation Laboratory Company. In such devices, the capillary tube is fixedly attached to a holding device which is threaded through a bore in a plexiglass chamber. The capillary tube is injected into a solution, such as a sodium potassium solution, which is intended to be aspirated in a chamber. The other end of the capillary tube, the capillary tip is positioned in close proximity to a source of air pressure which vents the air stream in the direction of the capillary tip at a right angle to the axis of the capillary tube. The air stream creates a vacuum which draws the solution through the capillary tube and out the capillary tip in atomized form in a chamber.
Optimum operation of such flame photometers requires an aspiration rate of one mililiter per 40 second .+-.5 second. In order to adjust the aspiration rate, the holding device having the capillary tube fixedly secured to it is rotated in the threaded bore of the chamber. This however, does not provide fine control of the lineal distance from the capillary tip to the output port of the source of the air current.
This is caused by the machining tolerances of the threads in the bore of the plexiglass chamber. This prevents the holding device and capillary tube from moving in perfect perpendicular alignment with the direction of the air stream. In such devices, turning the holding device, which holds the capillary tube, one or two degrees can change the aspiration rate by as much as 15 seconds. Thus such devices lack fine control of lineal displacement between the capillary tip and the air stream as a function of the rotational displacement of the holding device in the threaded bore of the plexiglass chamber. They therefore do not enable a user to closely control the aspiration rate of fluid flowing through the capillary tip.
A second problem inherent in such devices is caused by the rotation of the capillary tube about its axis as the holding device is rotated in the threads of the bore of the chamber. As the elongated member is rotated slightly the capillary tip which is fixedly attached to it rotates around its axis while at the same time moving closer to or farther away from the source of the air stream. The capillary tip thereby rotates in a circular pattern in a plane perpendicular to the axis of the capillary tube. This further detracts from the linearity of the movement of the capillary tip with respect to the direction of the air stream when the holding device which holds the capillary tube is rotated. Thus, these prior art devices do not enable a user to control in a very accurate manner the rate of aspiration of fluid in the chamber of the aspirator bowl.
Other prior art devices also seek to control the rate of aspiration of a solution. One such device is shown in U.S. Pat. No. 3,294,058 to Shriro. The Shriro Patent discloses a device for providing a fine spray which is drawn through a nozzle by a current of gas under high pressure. The position of the nozzle with respect to the orifice through which the source of gas under high pressure is vented is controlled by the movement of a mechanism which is threaded through a bore. The nozzle source therefore rotates longitudinally. This longitudinal rotation of the nozzle source, in order to adjust the distance between it and the source of high pressure gas, causes the same problems inherent in the flame photometers of the Instrumentation Laboratory Company described above. Thus, the Shriro device lacks fine control of the distance between the nozzle source of fluid to be aspirated and the source of high pressure gas and therefore lacks fine control of the rate of aspiration.